Almost all industrial processes for the polymerisation of fluoropolymers are based on the polymerization of a fluorinated monomer in a liquid phase in a mixed reaction vessel. Such polymerisation processes generally require reactants to be kept in intimate contact by means of a mixing flow. In such multiphase chemical reactions, especially tetrafluoroethylene (TFE) polymerisation, complicated phenomena of convection, diffusion and mass transfer among different phases (liquid/gas/solid) of the reactants are involved.
Suitable mixing devices are generally comprised in the polymerisation devices enabling notably:    1. homogenization, i.e. equalization of differences in concentration and temperature so as to avoid spatially dependent polymerization rates;    2. intensification of the heat transfer between the liquid and the heat-generating interface;    3. suspension of the solid in the liquid;    4. dispersion of the gaseous fluorinated monomer in the aqueous liquid phase.
If mixing is not effective, segregated regions can be found which generally increase by-products generated during the reaction.
In the case of the gas-liquid-solid reaction associated with the fluorinated monomer polymerisation in aqueous phase, it is generally understood that the reaction takes place between a reactant (i.e. the gaseous fluorinated monomer) in gas phase and the active sites in solid phase (i.e. the growing polymer chain), and that the liquid typically plays, inter alia, the role of withdrawing the enthalpy of the reaction.
In fluorinated monomer polymerization, the presence of segregated zones of solids, generally characterized by low thermal conductivity and where the gas can diffuse into the solid porosity, can disrupt temperature homogeneity and yield local temperature gradients. Such hot spots are to be avoided because they could engender thermal decomposition of monomers or polymers. When local overheating takes place in TFE polymerization, run-away reactions can lead, in addition to polymer sintering, to ignition and deflagration of monomeric TFE. For safety considerations, it is thus fundamental to achieve a homogeneous removal of the heat of polymerisation, especially in case of TFE.
Generally, to improve the homogeneity of the polymerisation medium in the reactor and avoid solid build-up and segregated zones:                fluorinated monomers polymerisation processes are carried out using high shear impellers with good pumping efficiency (to avoid solid settling); and        fully baffled reaction vessels (to obtain more uniform conditions) are used.        
Thus, U.S. Pat. No. 3,245,972 B (DUPONT DE NEMOURS) 12, Apr. 1966 discloses a process for obtaining a polytetrafluoroethylene (PTFE) molding powder, said process comprising polymerizing tetrafluoroethylene (TFE) in aqueous medium using stirring equipment with power consumption in the range 0.0004 to 0.002 kg m/sec/ml and with a ratio of power to discharge coefficient of at least 1.4. Different types of impellers are disclosed, such as, notably, a vertically disposed flat paddle, a stirrer with flat blades pitched at an angle of 15 degrees to the horizontal, a marine propeller and a bladed propeller having horizontal shear tips; a flat blade paddle stirrer arranged vertically with ratio of power to discharge coefficient of 3.4 is preferred.
U.S. Pat. No. 3,462,401 B (DAIKIN KOGYO CO., LTD) 19, Aug. 1969 disclose a process for the preparation of nearly spherical granular PTFE molding powder, said process comprising polymerizing in presence of a water insoluble organic liquid under vigorous stirring. The reaction device is a polymerisation vessel equipped with either an anchor type agitator or a four-vaned propeller in its bottom.
U.S. Pat. No. 5,760,148 B (DYNEON GMBH) 02, Jun. 1998 discloses the suspension polymerisation of TFE for obtaining uniform, compact and spherical grains of PTFE in a reactor equipped with a stirrer element generating both an axial flow component and a tangential flow component, with formation of a vortex in vicinity of the stirrer element, optionally by means of rotatable baffles without producing dead water zones.
One of the main disadvantages that arise when mixing a liquid/solid medium is the creation of a steady whirling or rotation of the mixture about the reaction vessel in a constant path, so that the actual intermingling of mixture is reduced and a void cone or vortex is generated around the shafts of the propellers.
It has been thus proposed of employing stationary baffle plates to break said excessive whirling motion.
Nevertheless, baffles have several drawbacks, notably:                in presence of sticky solids, said solids build-up in dead zone, typically behind the baffle.        when shear sensitive materials are present, locally stronger stirring must be avoided, since it increases the particle segregation with damage the material;        in TFE polymerization, the formation of uncontrolled solid polymer build-up is extremely dangerous, as said solid can directly react with gaseous TFE with no heat removal by the aqueous medium; consequent local overheating may cause TFE deflagration.        
In view of the above, alternative mixing devices have been proposed in the past in other domains of chemical industry for assuring better mixing of multiphasic liquid/solids systems.
Thus, U.S. Pat. No. 2,209,287 B (SIMPSON W. L.) 23, Jul. 1940 discloses a mixing device comprising a tank and a mixer having two coaxially mounted propellers of different sizes and means for driving said propellers in opposite directions.
U.S. Pat. No. 3,330,818 B (MONSANTO CO.) 11, Jul. 1967 relates to an improved process for the low pressure polymerization of olefins, especially ethylene, wherein a mixing system comprising two impellers mounted on coaxial separate shafts (one from the top, one from the bottom) rotating in opposite directions is used for forming a high turbulence zone, yielding a significant reduction in plating.
GB 2158727 (CHEM-PLANT STAINLESS LIMITED) 20, Nov. 1985 discloses a generally cylindrical mixer vessel comprising a central shaft extending vertically and carrying an agitator rotatable about the axis of said shaft in one direction and stirrer blades rotatable about the axis of the shaft in the opposite direction.
U.S. Pat. No. 6,252,018 B (BASF AG) 26, Jun. 2001 relates to a stirring system device particularly suitable for the emulsion polymerisation of ethylenically unsaturated monomers, said stirring system comprising multistage, very close-clearance stirring elements, which produce not only a tangential flow component but also an axial flow field.
It is also understood that polymer particles morphology and shape factors as well as comonomer distribution are strongly influenced by the mixing flow; particle size distribution and particle shape is notably affected by the mixing energy distribution, by the shear force, by the presence of dead zones, etc.
U.S. Pat. No. 6,262,209 (AUSIMONT S.P.A.) 17, Jul. 2001 discloses a process for producing modified PTFE by suspension polymerization, said process allowing obtaining at high productivity a product having improved processing features. Said polymerization process comprises polymerizing a mixture of TFE and a perfluoroalkyl vinyl ether and/or a perfluorodioxole comonomer at a pressure from 15 to 30 bar with a perfluorinated surfactant in the presence of a buffering salt, and a persulfate and a reducing agent as initiator. According to U.S. Pat. No. 6,262,209 (AUSIMONT S.P.A.) 17, Jul. 2001, provided that the comonomer is added during polymerization so as to maintain a constant incorporation rate and that a perfluorinated surfactant is used to improve comonomer uniformity, it is possible to obtain a modified PTFE suspension polymer powder using standard stirring equipments with good homogeneity in knived tapes (no transparent halos nor marbleization).
In the case of TFE suspension polymerization, the absence of baffles and the consequent oriented fluid flow generally generates oriented particle growth, leading to highly non-homogeneous particle having asymmetric shapes. Such asymmetric particles are not suitable for industrial use; subsequent particles seizing or further treatments are required for obtaining homogeneous powders for the industrial use.
There is thus a need in the art for a polymerisation process comprising a more efficient mixing system, assuring homogeneity in temperature and composition of the polymerisation medium, avoiding dead zones, enabling uniform turbulence intensity distribution.
There is also a need for a polymerization process enabling obtaining homogeneous mixing state at low and high shear conditions, without the use of mechanical baffles and thus avoiding the related dead zone problems.
There is finally a need in the art for a polymerisation process enabling manufacture of homogenous spherical powder particles, which have suitable morphology and shape factors which make them suitable for industrial use (e.g. for moulding) with no further grinding nor size reduction post-treatment.
All these problems and other are remarkably solved by the polymerisation process of the invention.